1. Field of the Invention
This invention pertains to improvements in heavy duty friction bearings. More particularly, this invention is directed toward improvement in the design and method of producing such bearings for demanding service, for example, as in rotary cone rock bits and the like.
2. Summary of the Prior Art
Many friction bearing designs are intended to be unidirectional, used to bear loads predominantly or exclusively in one direction. The "unloaded" side is then frequently used to reservoir lubricant in position as required to produce the hydro-dynamic lubricant film which bears the load on the remaining bearing area. Such designs are predominant in the cited example of the rotary cone rock bit.
One such bearing is described in U.S. Pat. No. 3,995,917 by Quinlan. In production, most of the length of the radial bearing shaft is undercut in relief. Subsequently, the "loaded" side is filled with arc deposited stellite which is then ground to the finished diameter. The unloaded and unfilled side provides a ready storage location for lubricant. The cooperating bore of the rotary cutter is undercut in a similar manner and filled with arc deposited aluminum bronze which is subsequently ground to diameter.
In a competing design, the unloaded side of the journal shaft is relieved for the storage of lubricant by means of an eccentric grinding operation. The shaft is used hardened and ground.
Yet another popular design features a hard metal filled relief on the loaded side of the journal shaft which is then ground full round. This bearing supports a matching bore with a composite surface. In production this bore is relieved longitudinally in narrow strips leaving unrelieved lands of similar dimension between. The reliefs are subsequently filled with arc deposited soft bearing metal, and the bore then ground to size. While the hard steel lands resist wear and support the load, the soft bars aid in load support and also serve to trap detritus by embedding it, thus slowing the degradation rate of the bearing.
Another bearing surface of similar nature with a unique method of production has been patented in this country, but is not practiced commercially. The steel bearing surface is deeply knurled, and subsequently overcoated with a fused deposit of softer bearing alloy. The bearing is then ground to size, producing a surface of small work hardened diamond shaped areas of steel within a grid of soft bearing alloy.
Another commercial rock bit bearing uses a split floating bushing of beryllium copper, or other bearing alloy, running between hardened steel bearing surfaces.
Rock bits are typically used until some part fails. Ideally, the bearings hold up till the cutting teeth wear out, and the bit may then be withdrawn without leaving metal parts in the well bore. Bearing failures are not uncommon, however, sometimes resulting in the down hole loss of rotary cones. A need exists, therefore, for further improvement in the bearing systems of rock bits aimed at lengthening the reliable life expectancy of such bearings in service.
The production of fine finishes and close running fits are important considerations in service longevity, not well supported by such practices as relieving the unloaded side of the supported by such practices as relieving the unloaded side of the bearing, or by the use of a floating bushing which also serves to double the running clearances. A bushing of any type gives a wider choice of materials with which to produce a long running bearing couple, but bushings also use up premium radial space which is a contraindication to their use in rock bits.
Another important area for improvement is in the nature of materials used in the construction of such bearings. Materials which are arc deposited are less than ideal choices being metalurgically non-homogeneous and of disordered microstructure.